The American Council on Education (ACE), which is the major coordinating body for all of the nation's higher education institutions, says that "attempting to convert GED test scores to GPA scores is like trying to compare apples to oranges." ACE provides more information about understanding GED test scores and college admissions here:
If you are asking your question because you are applying to a particular college, I suggest that you contact the admissions office and ask about their policy with regards to GED scores. Here's an example of the college that does accept GED scores, and that provides a telephone number students can use to have their scores submitted: http://www.mgc.edu/admissions/faq.cfm
I hope this information is helpful to you. Thanks for asking!